As a method for forming a color photograph or a color print, there is a method for forming an image, such as a picture or a character, on a photosensitive sheet by exposing the sheet. There are different types of photosensitive sheets, for example, a photosensitive sheet employing a multi-layer color development method, in which three layers of photosensitive emulsions with different color sensitivities are layered on a single supportive sheet thus forming a photosensitive member, a photosensitive sheet that employs a film in which each emulsion layer contains a pigment and a developing agent so that the film is capable of being exposed and developed simultaneously, and the like. A still another photosensitive sheet called Cycolor medium, as shown in FIG. 1, which employs, as a photosensitive material, microcapsules (cyliths) 3a, 3b, and 3c that contain different chromogenic substance and different photoinitiators. In the Cycolor medium 1, a thin supportive body 2 formed from, for example, polyester, is coated with a photosensitive material layer 4 containing numerous cyliths of a very small size. When exposed to light, cyliths harden so that only the cyliths of a specific color are activated, and the cyliths are ruptured by pressurization, and then developed, thereby forming an image of a predetermined color. Other photosensitive sheets have different color development principles, but need to be exposed to exposure light of the color of an image or its complementary color to form an image.
In widely used methods for exposing a photosensitive sheet, white light is split into three primary colors by a filter or the like, and images are formed using the individual primary colors, and then combined to form an image of predetermined colors or an image of their complementary colors on the photosensitive.
Another technology has recently been developed, as disclosed in Japanese patent application laid-open Nos. Hei 5-211666 and Hei 5-278260, in which LEDs or lasers that emit red light, green light and blue light are employed as light-emitting sources, and the light-emitting sources are controlled so that an image of predetermined colors is formed on a photosensitive sheet and the sheet is thereby exposed. However, in an exposure apparatus employing LEDs or lasers as light sources as disclosed in Japanese patent application laid-open Nos. Hei 5-211666 and Hei 5-278260, a lens system is employed to converge light emitted from the LEDs or lasers onto a medium. To control colors in the unit of dots, it is necessary to employ expensive optical systems that require a large installation space, such as a scanning optical system, a micro-lens array, and the like. The micro-lens array and lens groups constituting the scanning optical system have a loss in light transmission, so that only a portion of the light emitted from the LED or laser light sources reaches the photosensitive sheet (medium). Therefore, in some cases, LEDs are not sufficient to provide an amount of light required for exposure of a photosensitive sheet. In other cases, the printing rate must be reduced and the printing time must be increases in order to secure a sufficiently long exposure duration. In addition, an optical system employing lenses requires a large installation space, and is costly, so that a printing apparatus becomes large and costly.
Accordingly, it is an object of the invention to provide a small-size and low-cost exposure head and a small-size and low-cost printing apparatus that are capable of converging light radiated from semiconductor light sources, such as LEDs, onto a photosensitive sheet, and forming an image on the photosensitive sheet using high-intensity light, without employing an optical system that causes a cost increase and a size increase.
If high-intensity exposure light is obtained from the semiconductor light sources, a compact-size exposure head can be provided, and it becomes possible to provide light sources separately for individual colors and control each exposure duration. Thus, it is another object of the invention to provide an exposure apparatus capable of setting a suitable exposure duration for a photosensitive sheet having photosensitive materials with different exposure characteristics for individual colors. It is still another object of the invention to provide a low-cost and small-size exposure head and a low-cost and small-size printing apparatus that are capable of forming high-quality images with good color balance and reduced color distortion, at a fast rate. For example, in some Cycolor media as described above, the exposure characteristics of photosensitive materials may differ depending on colors. Thus, it is a further object of the invention to provide an exposure head and a printing apparatus that are capable of forming an image with good color balance and reduced color distortion and therefore producing a high-quality color print, on media, such as the Cycolor media.
It is a further object of the invention to provide an exposure head and a printing apparatus that are capable of preventing uneven color development caused by individual variations of semiconductor light sources, such as LEDs, and therefore forming on a photosensitive sheet an image with good color balance and no distortion.